Australian Prime Minister Tony Abbott visits at Seoul National Cemetery during his visit to South Korea on April 8. / Chung Sung-Jun Getty Images, Getty

by John Bacon, USA TODAY

by John Bacon, USA TODAY

The Australian prime minister expressed confidence Friday that a series of electronic pings detected from deep in the Indian Ocean are from the black boxes of the missing Malaysia Airlines jet, even as authorities acknowledged that the most recent pings were unrelated to the search.

Prime Minister Tony Abbott, speaking at a press conference in Shanghai, said the search area had been narrowed down to "within some kilometres" based on signals picked up by pinger locators.

"We have very much narrowed down the search area ... because we've now had a series of detections, some for quite a long period of time," Abbott said. "Nevertheless, we're now getting to the stage where the signal, from what we are very confident is the black box, is starting to fade."

The batteries that emit the signal are designed to last about 30 days. Beijing-bound Flight 370 veered sharply off course and vanished shortly after takeoff from Kuala Lumpur on March 8. Authorities believe the plane went down in the southern Indian Ocean, and the signals would put it about 1,000 miles off Perth, Australia.

Abbott said he would provide details of the search to Chinese President Xi Jinping. More than 150 of the 239 people aboard Flight 370 are from China, and the difficult search has been controversial there.

"We cannot be certain of success, but we can be certain of the effort and the professionalism that are being brought to the task," Abbott said at the press conference.

Australian Air Chief Marshal Angus Houston, who is leading that nation's Joint Agency Coordination Center, said Friday that an initial assessment of the most recent signals, detected Thursday by a Royal Australian Air Force AP-3C Orion aircraft, revealed they were apparently not related to the missing jet. The Australian Joint Acoustic Analysis Center is continuing to analyze all signal data, he said.

The Australian ship Ocean Shield was continuing to search for sounds, towing the U.S. Navy's TPM-25, a stingray-shaped listening device. The Navy also has provided a small underwater submarine, Bluefin 21, that can be used in the search, but it is a much slower process than the Ocean Shield's effort.

The underwater search zone has been trimmed greatly in recent days to about 500 square miles. It's a major improvement, but the area is still about the size of the city of Los Angeles.

"It is vital to glean as much information as possible while the batteries on the underwater locator beacons may still be active," Houston said.

Separately, a Malaysian government official said Thursday that investigators have concluded the pilot spoke the last words to air traffic control, "Good night, Malaysian three-seven-zero," and that his voice had no signs of duress. A re-examination of the last communication from the cockpit was initiated after authorities last week reversed their initial statement that the co-pilot was speaking different words.

The senior government official spoke on condition of anonymity because he wasn't authorized to speak to the media.